Cemented carbide articles such as cutting tools, mining tools, and wear parts are routinely manufactured from carbide powders and metal powders by the powder metallurgy techniques of liquid phase sintering or hot pressing. Cemented carbides are made by "cementing" hard tungsten carbide (WC) grains in a softer fully-dense metal matrix such as cobalt (Co) or nickel (Ni).
The requisite composite powder can be made in two ways. Traditionally, WC powder is physically mixed with Co powder in a ball mill to form composite powder in which WC particles are coated with Co metal. A newer way is to use spray conversion processing, in which composite powder particles are produced directly by chemical means. In this case, a precursor salt in which W and Co have been mixed at the atomic level, is reduced and carbonized to form the composite powder. This method produces powder particles in which many WC grains are imbedded in a cobalt matrix. Each individual powder particle with a diameter of 50 micrometers contains WC grains a thousand times smaller.
The next step in making a cemented carbide article is to form a green part. This is accomplished by pressing or extruding WC-Co powder. The pressed or extruded part is soft and full of porosity. Sometimes further shaping is needed, which can be conveniently done at this stage by machining. Once the desired shape is achieved, the green part is liquid phase sintered to produce a fully dense part. Alternatively, a fully-dense part is sometimes produced directly by hot pressing the powder. In a final manufacturing step, the part is finished to required tolerances by diamond grinding.
Cemented carbides enjoy wide applicability because the process described above allows one to control the hardness and strength of a tool or part. High hardness is needed to achieve high wear resistance. High strength is needed if the part is to be subjected to high stresses without breaking. Generally, cemented carbide grades with low binder levels possess high hardness, but have lower strength than higher binder grades. High binder levels produce stronger parts with lower hardness. Hardness and strength are also related to carbide grain size, the contiguity of the carbide grains and the binder distribution. At a given binder level, smaller grained carbide has a higher hardness. Trade-off tactics are often adopted to tailor properties to a particular application. Thus, the performance of a tool or part may be optimized by controlling amount, size and distribution of both binder and WC.
The average WC grain size in a sintered article will not, generally, be smaller than the average WC grain size in the powder from which the article was made. Usually, however, it is larger because of grain growth that takes place, primarily, during liquid phase sintering of the powder compact or extrudate. For example, one can start with 50 nanometer WC grains in a green part and end up with WC grains larger than 1 micrometer.
A major technical challenge in the art of sintering is to limit such grain growth so that finer microstructures can be attained. Thus, it is typical to add a grain growth inhibitor to WC-Co powder before it is compacted or extruded. The two most commonly used grain growth inhibitors are vanadium carbide (VC) and chromium carbide (Cr.sub.3 C.sub.2) with TaC and NbC used less frequently. However, the use of these additives presents some problems. First, both are particularly oxygen sensitive, and when combined with WC and binder metal in a mill, both tend to take up oxygen, forming surface oxides. Later, during the liquid phase sintering step, these oxides react with carbon in the mixture to form carbon monoxide (CO) gas. If extra carbon has not been added to the powder to allow for this consumption of carbon, then this results in the WC and Co forming brittle .eta.-phases, which ruins the article. If too much carbon has been added, so-called carbon porosity results, again ruining the article. Even if just the right amount of carbon has been added, the evolution of CO gas itself can lead to unacceptable levels of porosity. High oxygen levels in powder compacts or extrudates lead to major problems during their sintering.
The present invention is premised on the realization that grain growth inhibitors, including vanadium carbide, chromium carbide, niobium carbide and tantalum carbide can be incorporated into a cobalt/tungsten cobalt carbide matrix during the formation of the cobalt/tungsten cobalt carbide matrix. More specifically, the present invention is premised on the realization that suitable salts of vanadium, chromium, tantalum, niobium or mixtures thereof can be combined with cobalt and tungsten compounds, dissolved into solution, and spray dried to form precursor compounds. In turn, the precursor compounds can be carburized using a two-step process to form tungsten carbide embedded in cobalt matrix, along with the carbides of vanadium, chromium, tantalum and/or niobium, while retaining the fine grain structure in the powder.
The carburization process requires a two-step process. In the initial process a relatively low carbon activity gas formed from carbon monoxide and carbon dioxide are used at relatively low temperatures --about 750.degree. C. to 850.degree. C. This is continued until the tungsten is completely reacted to form tungsten carbide. This will leave the grain growth inhibitor composition as an oxide. The carburization is then continued using a gas having a higher carbon activity, specifically a combination of hydrogen and a hydrocarbon at a higher temperature, about 850.degree. C. to 950.degree. C., for no more than one hour. This will quickly cause the grain growth inhibiting composition to change from an oxide to a carbide without adversely affecting the previously-formed tungsten carbide/cobalt matrix. This allows the grain growth inhibitor to be directly formed with the cobalt/tungsten carbide matrix providing for more uniform distribution, less oxide formation, less oxygen sensitivity, and retention of fine grain size. This also reduces processing steps.
The objects and advantages of the present invention will be further appreciated in light of the following detailed description.